The importance of nuclear spin diffusion as an angularly independent relaxation mechanism controlling the matrix ENDOR response of radicals in molecular crystals

In the generalized matrix ENDOR line shape model [P.A. Narayana et al., J. Chem. Phys., 67, 1990 (1977)] the ENDOR intensity is controlled by the nuclear spin lattice relaxation rate formulated as an angularly dependent electron-nuclear dipolar contribution and an angularly independent intrinsic bulk nuclear relaxation contribution. Comparison of this model with single crystal data [L. Kevan et al., J. Chem. Phys., 70, 5006 (1979)] showed that the angularly independent relaxation rate must be more rapid than the intrinsic bulk nuclear relaxation rate. Here we identify the angularly independent relaxation rate with nuclear spin diffusion and apply this interpretation quantitatively to data on methyl radicals in lithium acetate dihydrate crystals and to the radiation generated CH3CH(NH3)COOH- radical in alanine at 77 K